In image data processing for images captured by an input device such as a video camera and a digital camera, a high frequency portion of the image data is enhanced. This enhancement process is a type of edge-enhancement and is also called an aperture correction process. For example, referring to FIG. 1, a prior-art system includes an image-capturing device 301 such as a video camera and a digital camera generates image data. The image data includes a Cyan/Yellow (Cy/Ye) signal as well as a Magenta/Green (Mg/G) signal which are respectively an output ratio between elements responsive to Green/Red and Blue/Green in the image-capturing device. In response to the above signals, an interpolation matrix unit 302 outputs a Red (R) signal, a Green (G) signal, a Blue (B) signal as well as light intensity signal Y. The RGB color signals are white balanced by a white balance unit 303, then gamma converted by a gamma-conversion unit 304 and lastly processed by a subtractive primaries matrix unit 305 so as to generate Cr and Cb signals. The Cr signal is a difference betweeen R and Y signals while the Cb signal is a difference between B and Y signals. The Cr and Cb signals are outputted to output devices such as a display device. On the other hand, the light intensity signal Y is processed by an aperture correction device 306.
Still referring to FIG. 1, a prior-art aperture correction device 306 corrects degraded data which includes a high frequency portion representing patterns such as a continuously repeated pattern and an abrupt intensity change pattern. The image data is generally more adversely affected as an aperture size increases. The aperture correction is generally more effective at a low resolution level such as 100 dpi or 100,000 charged-couple device (CCD) elements. The aperture correction device 306 includes a high-pass filtering unit 307 for extracting a high frequency portion of the intensity light signal Y. A coring unit 308 reduces noise of the extracted high frequency portion of the light intensity signal within a predetermined core range. In other words, between the two threshold values, the noise is reduced to zero. A gain adjust unit 309 amplifies the noise reduced signal according to a predetermined gain value. A limiting unit 310 includes a function which has a predetermined maximal output value for any input value above a predetermined positive threshold value as well as a predetermined minimal output value for any input value below a predetermined negative threshold value. The limiting unit 310 outputs a limiting output signal according the above-described function. The aperture correction device 306 finally outputs an enhanced high-frequency portion light intensity signal by adding the limiting output signal to the original relative intensity value. The enhanced high-frequency portion light intensity signal is converted by another gamma conversion unit 311 before outputting to an output device such as a display unit.
Now referring to FIG. 2, another prior-art aperture correction device 400 outputs individual enhanced high-frequency portion light intensity signals for each of the RGB signals. After image data is generated by an image-capturing unit 401, an interpolation matrix unit 402 converts the image data into RGB signals. A white balance unit 403 balances each of the RGB signals. The aperture correction device 400 corrects degraded data which includes a high frequency portion representing patterns such as a continuously repeated pattern and an abrupt intensity change pattern. The aperture correction device 404 includes a set of high pass filtering units 404R, 404G and 404B for extracting a high frequency portion of each of the RGB signals. A set of coring units 405R, 405G and 405B reduces noise of each of the extracted high frequency portion of the RGB signals within a predetermined core range. In other words, between the two threshold values, the noise is reduced to zero. A set of gain adjust units 406R, 406G and 406B each amplifies respective one of the noise reduced RGB signals according to a predetermined gain value. A set of limiting units 407R, 407G and 407B includes a function which has a predetermined maximal output value for any input value above a predetermined positive threshold value as well as a predetermined minimal output value for any input value below a predetermined negative threshold value. The limiting units 407R, 407G and 407B each output a limiting output signal according the above-described function. The aperture correction device 400 finally outputs an enhanced high-frequency portion RGB signals by adding the limiting output signal to the corresponding original RGB signal. The enhanced high-frequency portion RGB signal is converted by a set of gamma conversion units 408R, 408G and 408B before outputting to an output device such as a display unit.
Now referring to FIGS. 3A through 3D, the above-described problem is illustrated in graphs. FIG. 3A illustrates a high frequency portion of an input signal, and FIG. 3B shows the result of an edge enhancement process. The edge enhanced or aperture corrected signal undergoes a gamma conversion process for a given output device. Notice that since the gamma correction is a non-linear conversion process, the aperture corrected signal is not uniformly converted. FIG. 3D shows that the gamma correction process generates uneven results in an input signal to an output device such as a display unit. Because of the uneven input signal, the display unit shows a somewhat darker area as an average intensity level as indicated by a dotted line.
The above described prior-art aperture correction devices performs uniform and fixed corrections on a high-frequency portion of each signal independent of the relative intensity level of the signal. Since these uniform and fixed corrections overcompensate a low relative intensity signal, the overall output image appears to be adversely affected. In order to solve the overcompensation problem, one prior-art reference such as Japanese Laid Patent Publication No. 8-9199 discloses the use of variable coefficients in the correction process. In other words, the disclosures of this prior-art reference teach the use of a low coefficient value for a low intensity signal.
An additional problem in visualizing the corrected image data still exists despite the above-described improved aperture correction. Although the gamma-corrections are necessary for correcting the image data according the gamma characteristics of a given output device, the gamma-corrections often yield washed-out appearance in high-light portions of the image. To avoid the undesirable appearance, the gamma-corrections have been modified in the actual application so as to treat the image data in the high-light portions in a different manner. Because of this special treatment in the gamma-corrections, the intensity in the image data and the output device such as a display monitor is often not linear. In other words, even though the average light intensity value of the image data after the gamma-corrections is substantially the same as that before the gamma-corrections, the appearance of outputs such as a display image on a display monitor becomes darker or lighter than the original. The discrepancy in perception also occurs when the gamma-corrections are performed before the aperture corrections, and the perceptual discrepancy remains to be improved.